r336028 changed next_msg to a char * from char [] of fixed size. Change

2nd argument of vsnprintf() to get the strlen of next_msg so that the
appropriate size is used.

Found with gcc.

/usr.bin/top/display.c: In function 'new_message':
/usr.bin/top/display.c:963:31: error:
argument to 'sizeof' in 'vsnprintf' call is the same expression as the
destination; did you mean to provide an explicit length?
[-Werror=sizeof-pointer-memaccess]
     vsnprintf(next_msg, sizeof(next_msg), msgfmt, args);

Reviewed by:	daichi

usr.bin/top/display.c

@@ -960,7 +960,7 @@ new_message(int type, const char *msgfmt, ...)
     va_start(args, msgfmt);
 
     /* first, format the message */
-    vsnprintf(next_msg, sizeof(next_msg), msgfmt, args);
+    vsnprintf(next_msg, strlen(next_msg), msgfmt, args);
 
     va_end(args);